1. Field of the Invention
The present invention relates to an offset amount measuring method of and apparatus for measuring an offset amount which is a shift amount in the radial direction between radial positions in a plurality of recording layers, in an information recording medium, such as a DVD and a CD (Compact Disc) having a two-layer structure or multilayer structure, for example.
2. Description of the Related Art
In an information recording medium, such as a CD-ROM (Compact Disc-Read Only Memory), a CD-R (Compact Disc-Recordable), and a DVD-ROM, for example, as described in Japanese Patent Application Laid Open NO. 2000-311346 and NO. 2001-23237, etc., there is also developed an information recording medium, such as a multilayer type or dual layer type optical disc, in which a plurality of recording layers are laminated or pasted on the same substrate. Then, on an information recording apparatus, such as a DVD recorder, for performing the recording with respect to the dual layer type, i.e., two-layer type, optical disc, laser light for recording is focused or condensed on a recording layer located on the front (i.e. on the closer side to an optical pickup) as viewed from the irradiation side of the laser light (hereinafter referred to as an “L0 layer”, as occasion demands) to thereby record information into the L0 layer in an irreversible change recording method by heat or a rewritable method. Moreover, the laser light is focused or condensed on a recording layer located on the rear of the L0 layer (i.e. on the farther side from the optical pickup) as viewed from the irradiation side of the laser light (hereinafter referred to as an “L1 layer”, as occasion demands), through the L0 layer or the like, to thereby record information into the L1 layer.
In such a two-layer type optical disc, if the data is recorded into the L1 layer, it is necessary to irradiate the laser light through the L0 layer. In this case, the record data may be recorded in the L0 layer, or may not be recorded. As described above, the recording state in the L0 layer is not necessarily standardized, which causes a change in the state of the laser light with which the L1 layer is irradiated. Thus, a method is also considered by the present inventors or the like in which the L0 layer is made in a recorded state to thereby properly record the record data into the L1 layer.
However, in preparing such a two-layer type information recording medium, the L0 layer and the L1 layer are formed by different stampas or cutting machines and are pasted in the end. Thus, in the L0 layer and the L1 layer, there arises an eccentricity due to a pasting error, to a greater or lesser extent. Alternatively, since the L0 layer and the L1 layer are formed by different stampas or cutting machines, there arises deviation in a track pitch in each recording layer, to a greater or lesser extent.
In particular, even if the above-mentioned eccentricity and the deviation are caused, it is unclear to what extent they are. Moreover, it is also unclear whether or not there are countermeasures to the eccentricity and the deviation, and whether or not the countermeasures are good if some countermeasures are taken. In addition, even if it is tried to measure the eccentricity and the deviation with respect to the information recording medium before the record information is recorded by a user, it is unclear where is the address part targeted for the measurement in an unrecorded state. In other words, practically, it is hardly possible or impossible to measure the eccentricity and the deviation, and to judge verification of whether or not the countermeasures are good and whether or not the countermeasures taken are effective, if some countermeasures are further taken.
As a result, there is such a problem that the eccentricity and the deviation cause a shift or deviation in a position in another recording area of the L1 layer which is associated with one recording layer of the L0 layer by position information, such as a pre-format address, for example, so that the another recording area of the L1 layer is not necessarily irradiated with the laser light through one recording area which is in the recorded state in the L0 layer.